1. Field of the Invention
The present invention relates to a fuel cell system equipped with a fuel cell that generates electric power through an electrochemical reaction between an oxygen-containing gas, which is supplied to a cathode, and a fuel gas, which is supplied to an anode, an oxygen-containing gas supply device for supplying the oxygen-containing gas to the fuel cell, and a fuel gas supply device for supplying the fuel gas to the fuel cell. The present invention further relates to method for stopping power generation in such a fuel cell system.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell incorporates a membrane electrode assembly (electrolyte electrode assembly) (MEA) including an anode and a cathode disposed on respective both sides of an electrolyte membrane made up from a polymer ion exchange membrane, and a pair of separators between which the membrane electrode assembly is sandwiched. A fuel gas flow field for supplying a fuel gas to the anode is formed between one of the separators and the membrane electrode assembly, and an oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode is formed between the other of the separators and the membrane electrode assembly.
Normally, a plurality of such fuel cells are stacked to form a fuel cell stack. The fuel cell stack is incorporated in a fuel cell electric vehicle in association with various auxiliary devices such as an oxygen-containing gas supply device, a fuel gas supply device, and a coolant supply device, etc., thereby to form a vehicular fuel cell system.
In such a fuel cell system, as noted above, a solid polymer electrolyte membrane is used. In the solid polymer electrolyte membrane, it is necessary to retain a suitable amount of moisture for the purpose of assuring favorable ion conductivity. For this reason, by humidifying beforehand the oxygen-containing gas supplied to the cathode side of the fuel cell, or the fuel gas supplied to the anode side of the fuel cell, drying-out of the solid polymer electrolyte membrane is prevented and a desired humidified state of the solid polymer electrolyte membrane is maintained.
For example, in the fuel cell power generation system disclosed in Japanese Laid-Open Patent Publication No. 2005-268117 (hereinafter referred to as “conventional technique 1”), as shown in FIG. 6, there are provided a supply passage 2a that supplies an oxygen-containing gas to a fuel cell 1a, a discharge passage 3a in which an oxygen-containing off gas flows, which is discharged from an oxidant electrode of the fuel cell 1a, and an off gas return passage 4a that communicates between the supply passage 2a and the discharge passage 3a, and which returns at least a portion of the humid oxygen-containing off gas discharged from the fuel cell 1a back to the supply passage 2a. 
In the supply passage 2a, a rotational oxygen-containing off gas conveyance drive source 5a is installed at a location downstream from a region where the supply passage 2a and the off gas return passage 4a merge. Owing thereto, the pre-reaction oxygen-containing gas supplied to the fuel cell 1a is humidified by the post-reaction humid oxygen-containing off gas, and therefore a dedicated humidifying device (humidifier) can be dispensed with.
Furthermore, after the pre-reaction oxygen-containing gas and the post-reaction oxygen-containing off gas have been combined to result in a combined fluid, since the oxygen-containing off gas is passed through the oxygen-containing off gas conveyance drive source 5a, the combined fluid is positively mixed, so that diffusive mixing thereof can be enhanced.
On the other hand, with the above fuel cell system, water is generated during power generation, and when power generation is stopped, it is easy for such generated water to be retained on a downstream side from the oxygen-containing gas flow field and the fuel gas flow field. Additionally, at a time that the fuel cell is stopped, if scavenging by air is carried out in the oxygen-containing gas flow field and the fuel gas flow field, when the fuel cell is restarted, deterioration of the cathode is induced, in particular, due to a high potential on the downstream side of the oxygen-containing gas flow field.
Thus, a method for stopping power generation of the fuel cell is known, as disclosed, for example in Japanese Laid-Open Patent Publication No. 2003-115317 (hereinafter referred to as “conventional technique 2”). With such a method, as shown in FIG. 7, the open/closed state of a first flow passage switching valve 1b is opened so as to connect an air circulation passage 2b and an air compressor 3b, and to block access to the air inflow side. Further, the open/closed state of a second flow passage switching valve 4b is opened to allow an air off gas, which is discharged from a fuel cell 5b, to flow through the air circulation passage 2b, and to block access to a side where the air off gas is released to atmosphere.
For this reason, in the fuel cell 5b, new outside air is not supplied, and the air off gas, which is discharged from the cathode of the fuel cell 5b, is circulated in a closed circuit from the fuel cell 5b, a second air shutoff valve 6b, the second flow passage switching valve 4b, a dehumidifier 7b, the first flow passage switching valve 1b, the air compressor 3b, a first air shutoff valve 8b, and then back to the fuel cell 5b. 
Consequently, oxygen contained in the air off gas is consumed, and the oxygen concentration within the air off gas is reduced. Accordingly, after power generation is stopped, even if cross leakage occurs, almost no reaction takes place between hydrogen and oxygen, and the solid polymer electrolyte membrane can be protected.